1. Field of the Invention
The present invention relates to a laminated film for electrophotography on which images are directly formed (recorded) by an electrophotographic image forming device. In particular, the invention relates to a laminated film for electrophotography that can be used by; noncontact or direct-contact information media storing personal information and image information such as cash cards with full-face pictures, identification cards for a company's staff, student identification cards, membership cards, address cards, various driving license cards and various certificates; image sheets and image display panels for personal verification used in hospitals; and labels.
2. Description of the Related Art
Measures for forming a large quantity of images having an identical quality by various printing methods, such as intaglio printing, relief printing, planographic printing, photogravure printing and screen printing are known in the art, accompanying development of image forming technologies. Such printing methods have been frequently used for surface printing of information media such as IC cards, magnetic cards, optical cards or combinations of these information media, in which the media carry predetermined information and are able to communicate with external devices by a contact or non-contact mode.
However, for example, the above-mentioned screen printing method requires, a plurality of printing plates, the number of which corresponds to a number of images to be printed, and in the case of color printing, additional printing plates, the number of which corresponds to the number of colors used therein, are further required. Therefore, such printing methods are not suitable for printing respective identification information for individuals (such as full-face pictures, names, addresses, dates of birth and various certificates).
Currently, the most common image forming methods that account for the above problems are methods using printers which employ sublimation or melting thermal transfer methods using ink-ribbons. However, although these methods enable easy printing of personal identification information, these methods still have a problem in that resolution decreases when printing speed is increased, and printing speed decreases when resolution is improved.
In contrast to the above, in an electrophotographic image forming (printing) method, a surface of a image supporting member is uniformly electrified, exposed with light in accordance with image signals, an electrostatic latent image is formed in accordance with a difference in potential between exposed portions and non-exposed portions, and then a visible image (toner image) is formed on the surface of the image supporting member by electrostatic development using colored powders called toners (image forming materials) having the same (or opposite) polarity as the polarity of the electrified surface. When color images are to be formed, visible color images are formed by repeating the above steps multiple times, or by aligning a plurality of image forming devices, and these images are transferred to and fixed on (fixing: melting of the colored powders mainly by heating, and solidifying the melted colored powders by cooling) an image recording medium.
Since the electrostatic latent images are electrically formed on the surface of the image supporting member by image signals in the electrophotography method, not only can one image be formed any number of times, but image forming, which can easily correspond to different images, is possible. The toner images on the surface of the image supporting member can be almost completely transferred onto the surface of the image recording medium, and trace amounts of toner images remaining on the surface of the image supporting member can be readily removed with a resin blade and brush. Consequently, it is possible to easily produce prints corresponding to many items in small scale production.
The toner is generally formed by melt-mixing a thermal-fusible resin and pigment, and in some cases additives such as an antistatic agent, and pulverizing the kneaded product into fine particles. Resolution of the electrostatic latent image in the electrophotographic method is rather high as compared with that of the finely pulverized toner, and sufficiently high resolution can be expected as compared with resolution obtained by screen printing methods or ink ribbon thermal transfer printing methods.
By using color toners corresponding to four primary colors of cyan, megenta, yellow and black and by mixing these toners, it is theoretically possible to reproduce the same kind of colors in a same extent as in printing. Moreover, since toner resins can be relatively freely blended with pigments for forming the color toners, image concealability with the toner can be readily increased.
Heat I resistance and light stability of the image recording member in outdoor uses have been seldom studied. Particularly, the thermal-transferred images using dyes as coloring materials fade in cases when, for example, a driving license card is exposed to direct sunlight in a car. However, pigments which are excellent in light stability and exhibit various colors of cyan, magenta, yellow and black, are used in the color toners as outputs of the color images by the electrophotographic method. Accordingly, light stability of the image recording member in the electrophotographic method is considered to be sufficiently excellent. Likewise, heat resistance of the image recording member is considered to be sufficient for use outdoors if heat resistant toners are selected.
Substrates (cores) of various cards that are currently most frequently used are vinyl chloride sheets because the material is excellent in both printing characteristics and compatibility with embossing (concave and convex processing for letters). However, when being disposed of due to expiration of limitation period and the like, the vinyl chloride sheet has a problem of generating dioxins when disposed of by incineration with incinerators and the like. Thus, various sheet films are beginning to be used now as substitutes for the vinyl chloride from the view point of compatibility with the environment.
A conventionally-used biaxially-drawn PET (polyethylene terephthalate) film may be used on the premise that no embossing will be applied thereto. However, since embossing is often indispensable in order to keep the conventional function of the card, ABS resin films and polyolefin resin films that are softened at a relatively low temperature, a denatured PET resin called PETG, and integrated films of the denatured PET resin with a PET film, amorphous PET film or polycarbonate resin film are currently used.
Examples of various cards, which have been printed using the electrophotographic device are as follows.
For example, in addition to various kinds of personal information, an invisible bar-code is printed on a vinyl chloride sheet having a thickness of 250 μm or a polyester sheet having a thickness of 260 μm by electrophotography, and an over-film is disposed on a printed surface and is laminated with a heat press machine (see Japanese Patent Application Laid-Open (JP-A) No. 2001-92255).
However, because a friction coefficient between the sheets is overly large, the transfer property of the sheet becomes poor and the sheets adhere to each other. Consequently, the electrophotographic device may be interrupted. Moreover, when insulators (sheets) have a thickness of 250 μm or more as described above, image defects may increase due to the insufficient transfer of the image forming material (toner). Moreover, resin films having a relatively low softening point become sticky during the fixing process when such film is printed using the electrophotographic device, since the fixing temperature is higher than the softening temperature of the resin film, causing a trouble of generating winding jams in the fixing apparatus. Furthermore, the fixing apparatus may be unnecessarily damaged by the edge of the sheet when the image forming material is offset on the fixing device or when fixing of the sheet with a thickness of 250 μm is continued.
Alternatively, personal identification information is printed on a light permeable sheet, and the image may be printed in a mirror image (see JP-A No. 11-334265). However, JP-A No. 11-334265 merely teaches that a polyvinyl chloride sheet may be used, although at least a part of the light permeable sheet preferably comprises a biaxially drawn polyester film, or the light permeable sheet is preferably a biaxially drawn polyester film comprising ABS or polyester film.
Accordingly, a resolution comparable to that of the thermal transfer method cannot be obtained due to poor transfer of image forming materials onto the surface of the film, since the film simply made of an insulation material in JP-A No. 11-334265. Furthermore, in the printing machine emphasizing improvements of productivity, much loss and waste inevitably arise in order to comply with urgent production or small scale production of many kinds of articles when different printing works for one or several persons are required, because the laminated sheet used is in a roll form.